The sense of smell resides in the nostrils, which have no relation to the work of breathing. No fish breathes through its nostrils, and only in a few of the lowest forms (hagfishes) does the nostril pierce through the roof of the mouth. In the bony fishes the nostril is a single cavity, on either side, lined with delicate or fringed membrane, well provided with blood-vessels, and with nerves from the olfactory lobe. In most cases each nasal cavity has two external openings. These may be simple, or the rim of the nostril may be elevated, forming a papilla or even a long barbel. Either nostril may have a papilla or barbel, or the two may unite in one structure with two openings or with sieve-like openings, or in some degenerate types (_Tropidichthys_) with no obvious openings at all, the olfactory nerves spreading over the skin of a small papilla. The openings may be round, slit-like, pore-like, or may have various other forms. In certain families of bony fishes (_Pomacentridæ_, _Cichlidæ_, _Hexagrammidæ_), there is but one opening to each nostril. In the sharks, rays, and chimæras there is also but one opening on either side and the nostril is large and highly specialized, with valvular flaps controlled by muscles which are said to enable them "to scent actively as well as to smell passively." In the lancelet there is a single median organ supposed to be a nostril, a small depression at the front of the head, covered by ciliated membrane. In the hagfish the single median nostril pierces the roof of the mouth, and is strengthened by cartilaginous rings, like those of the windpipe. In the lamprey the single median nostril leads to a blind sac. In the _Barramunda_ (_Neoceratodus_) there are both external and internal nares, the former being situated just within the upper lip. In all other fishes there is a nasal sac on either side of the head. This has usually, but not always, two openings. There is little doubt that the sense of smell in fishes is relatively acute, and that the odor of their prey attracts them to it. It is known that flesh, blood, or a decaying carcass will attract sharks, and other predatory fish are drawn in a similar manner. At the same time the strength of this function is yet to be tested by experiments. [Illustration: FIG. 83.--Dismal Swamp Fish, _Chologaster cornutus_ Agassiz. Supposed ancestor of _Typhlichthys_. Virginia.] [Illustration: FIG. 84.--Blind Cavefish, _Typhlichthys subterraneus_ Girard. Mammoth Cave, Kentucky.]